This invention is in the field of motor control, and is more specifically directed to control of voice coil motors as used in computer disk drive controllers.
As is evident in the industry, the magnetic disk drive remains the dominant technology for mass read/write storage in modern computers, including both desktop workstations and also portable “laptop” computers. Magnetic disk drives are now also popular in smaller scale portable systems, such as portable audio systems and players.
Modern disk drives typically include a “spindle” motor and a “voice coil” motor. The spindle motor spins the magnetic disks during operation, so that sectors at a given radius of the disk pass by the data transducer, or read/write “head”. The “voice coil” motor radially positions the data transducer at the radial positions of the magnetic disk surface that correspond to the track locations to which data are being written or from which data are being read. Typically, the data transducer is at the end of a positioning arm that pivots across the surface of the spinning magnetic disk, from a pivot point outside of the circumference of the magnetic disk, so that pivoting of the positioning arm changes the radial position of the data transducer over the magnetic disk surface. The voice coil motor controls the pivoting of the positioning arm, and thus the track location of the data transducer.
Voice coil motor controller circuitry generally provides drive signals to the voice coil motor through a pair of output drivers, typically including a positive-side driver and a negative-side driver, connected on opposite sides of the voice coil motor. In operation, the positioning arm is pivoted in one direction by the positive side driver driving current through the voice coil motor to the negative side driver, and pivoted in the opposite direction by the negative side driver driving current through the voice coil motor to the positive side driver.
U.S. Pat. No. 6,374,043, issued Apr. 16, 2002, assigned to Texas Instruments Incorporated and incorporated herein by this reference, describes a voice coil motor driver and controller. This conventional voice coil motor driver includes a pair of driver transistors for each of the positive side and negative side voice coil motor drivers, and controls these driver pairs to operate in continuous and linear Class-AB mode around the driver crossover point.
Dual-mode voice coil motor control is known in the art. By way of further background, U.S. Pat. No. 5,838,515 describes a dual mode voice coil motor driver that operates in a pulse-width-modulated (“Class D”) mode and also in a linear mode. As well known in the art, the “track following” operating mode of the voice coil motor maintains the data transducer at a desired track location, and the “track seek” operating mode moves the positioning arm from one track location to another. This reference describes that power dissipation in the voice coil motor drivers is reduced by switching the voice coil motor driver from a linear mode into a pulse-width-modulated mode at the onset of a deceleration phase of the seek trajectory. This reference discloses that the driver is switched back to linear mode as the positioning actuator moves the read/write head toward its intended destination track, so that the driver is operated in a linear, constant current mode during track following.                It is believed that other disk drive systems may associate these two voice coil motor drive modes (linear and pulse-width-modulated drive) with different disk drive operations. From the standpoint of the voice coil motor controller, the availability of the two drive modes provides the disk drive system implementer with a great deal of flexibility.        
It has been discovered, in connection with this invention, that the transition of the voice coil motor driver from a pulse-width-modulation mode to the linear operation mode can produce discontinuities in the current through the voice coil motor. These current discontinuities result in errors in the positioning arm motion, which are especially undesirable when occurring in the deceleration phase of the track seek operation. Track settle failures can result as the data transducer is approaching the desired disk track, causing increased seek settling times and, in severe cases, errors in data write and data read operations.